Why does the catcher pass a pitched ball to umpire if it hits the ground?
In professional baseball, anytime a ball strikes something foreign with significant speed, an umpire must examine it for blemishes, cuts, etc. before putting back into play. Thus, when a pitched ball hits the ground, the umpire will examine it before putting it back into play. The catchers know this and thus do it automatically. If there are baserunners, the umpire will call time before accepting the ball.